The electron spin resonance (ESR) spectra of equimolar complexes of Cu(II) with plasma albumins from various species have been examined at 77 degrees K. Below pH 7 canine and porcine albumin-Cu(II) complexes showed poorly resolved copper hyperfine splittings in contrast to four well-separated g" lines observed in cupric derivatives of bovine and human albumins. At pH 11 nitrogen superhyperfine splittings were detected in all of the above complexes, indiating the involvement of three nitrogens in chelation with copper. A topographic study of human serum albumin (HSA) has been initiated with the aid of a spin label analog of isothiocyanate. From its bound ESR spectrum, one strongly immobilized binding site (2T" equals 64 Gauss) has been detected. The effect of alterations in the conformation of HSA induced by changes in such parameters as pH have been examined.